Transferred plasma arc torch

ABSTRACT

Herein disclosed is a transferred plasma arc torch capable of obtaining an excellent cut face having a small angle of inclination even in the case of cutting a thick plate. In the torch, the tip of an electrode holder (2) holding an electrode (3) fixedly is enclosed by a nozzle (1) forming a tapered working gas passage (4) together with the electrode holder (2). The nozzle (1) has such a chamber (7, 7a, 7b or 7c) inside of a throat (1a) formed in the tip of the nozzle (1) as faces the electrode (3). The following relations are satisfied if the throat diameter of the nozzle (1) is designated at d and if the diameter and axial length of the chamber are designated at D and H, respectively: 1.5≦D/d≦10; and 0.75≦H/D≦5. A swirler (6) for generating a swirling flow is disposed upstream of the working gas passage (4).

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a transferred plasma arc torch and,more particularly, a transferred plasma arc torch capable of obtainingexcellent cut face even in the case of cutting a thick plate.

BACKGROUND OF THE INVENTION

One transferred plasma arc torch according to the prior art is shown inFIG. 1. This transferred plasma arc torch of the prior art has its torcha tapered at its portion facing an electrode c which is held by anelectrode holder b. The working gases in the torch are guided to passthrough a working gas passage e, which is in the shortest gap around theelectrode holder b, and are then accelerated in the nozzle a having agradually reduced effective area, until they reach the throat f of thenozzle a, which has a diameter d and a length l. From this throat f, thegases are discharged to the outside at a velocity equal to their sonicvelocity.

In the case of the transferred plasma arc torch of a metal plate usingthe aforementioned nozzle a of the prior art, the cut face is highlyfrequently inclined from the start of the cutting even if the nozzle aused is new.

According to our investigations, it has been found that the major causesfor the aforementioned phenomena are, as follows:

Due to a minute displacement between the center lines of the electrode cand the nozzle a, the gap between the electrode c and the nozzle abecomes uneven so that the flow velocity of the working gases passingthe working gas passage e in that gap becomes asymmetrical with respectto the center line of the nozzle a to establish serious velocitydifferences. The working gases having such asymmetrical velocitydistribution will establish disturbances such as vortexes when they flowinto the mixing chamber located downstream of the electrode c. Thesedisturbances are not damped but released as they are to the outsidebecause they are abruptly accelerated to the exit of the nozzle a havingthe gradually reduced effect area.

As a result, the velocity distribution remains asymmetrical with respectto the center line of the nozzle a even at the throat f of the nozzle aso that the directivity of the arc jet discharged to the atmosphere isdeteriorated to induce a deflection of the arc jet.

In order to examine these phenomena experimentally, we have observed thebehavior of the arc jet g to be discharged from the nozzle a, bygrinding out a portion of the tip of the electrode holder b of thecommercially available air plasma arc torch of 50 A grade to enlarge thedifference in the working gas velocity which is established by theworking gas passage e in the gap between the nozzle a and the electrodeholder b.

These experiments have been performed for several electrode holders andhave confirmed that the direction of deflection of the arc jet g and thedirection of the ground-out portion of the electrode holder b arealigned.

This means that the asymmetry, if any, of the gap to be formed betweenthe nozzle a and the electrode holder b due to the offset of the axeswill induce the deflection of the arc jet g thereby to cause aninclination of an angle θ at the cut face.

The aforementioned inclination of the cut face grows a serious problemin case the thickness of the plate to be cut is enlarged. In the case ofcutting a thick plate, a secondary working may be required, or still theworse the cut work cannot be used as the product, if the angle ofinclination of the cut face exceeds some extent.

In order to prevent occurrence of the aforementioned inclination of thecut face, it has been endeavored to raise the working accuracy of acomponent such as the arc body, the electrode holder or the nozzle.Despite of this endeavor, however, a complete solution has not beenobtained yet due to the practical limits in the working accuracy and theassembling accuracy.

SUMMARY OF THE INVENTION

The present invention has been conceived in view of the background thusfar described and has an object to provide a transferred plasma arctorch capable of obtaining an excellent cut face having a small angle ofinclination even in the case of cutting a thick plate.

In order to achieve the above-specified object, according to a firstmode of the present invention, there is provided a transferred plasmaarc torch of the type, in which the tip of an electrode holder holdingan electrode fixedly is enclosed by a nozzle forming a tapered workinggas passage together with said electrode holder, characterized: in thatsaid nozzle has such a chamber inside of a throat formed in the tip ofsaid nozzle as faces said electrode; and in that the following relationsare satisfied if the throat diameter of said nozzle is designated at dand if the diameter and axial length of said chamber are designated at Dand H, respectively:

    1.5≦D/d≦10;

    and

    0.75≦H/D≦5.

According to a second mode of the present invention, moreover, there isprovided a transferred plasma arc torch comprising a swirler disposedupstream of the working gas passage inside of said nozzle for generatinga swirling flow.

According to the transferred plasma arc torch of the aforementionedindividual modes of the present invention, the working gases flow in theform of the swirling flow into the working gas passage and areaccelerated downstream in the nozzle until they are discharged from thethroat of the nozzle, Even if, at this time, the aforementioned workinggas flow is disturbed due to the displacement of the axes of theelectrode holder and the nozzle, it is damped by the viscosity of theworking gases, while it passes through the chamber disposed in thenozzle to face the electrode, until it is released with an even velocitydistribution at the nozzle throat.

Thus, it is possible to obtain an excellent cut product having a cutface of small angle of inclination.

The aforementioned and other objects, modes and advantages of thepresent invention will become apparent to those skilled in the relevantart, from the following description to be made in connection withpreferred embodiments of the present invention with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a section showing an essential portion of the example of theprior art;

FIG. 2 is an explanatory view for explaining the used state in case thetip of the electrode holder of the example of the prior art is partiallyremoved;

FIG. 3 is a section showing an essential portion of a specificembodiment of the present invention;

FIG. 4 is a section taken along line II--II of FIG. 3;

FIGS. 5, 6 and 7 are sections showing essential portions of otherspecific embodiments of the present invention, respectively;

FIG. 8 is a section showing the tip of a transferred plasma arc torch asone specific embodiment of the present invention; and

FIGS. 9A and 9B are diagrams showing and comparing the agings of thebevel angles of the example of the prior art and the specific embodimentof the present invention, respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Several specific embodiments of the present invention will be describedwith reference to FIGS. 3 to 9B.

FIG. 8 shows the overall structure of the transferred plasma arc torchaccording to the present invention. Designated at reference numeral 2 isa cylindrical electrode holder which holds an electrode 3 fixedly at theaxial portion of its tip. Moreover, this tip of the electrode holder 2is enclosed at a gap by a nozzle 1 to form this gap at the inside of thenozzle 1 into a tapered working gas passage 4. On the other hand, theinside of the aforementioned electrode holder 2 and the outside of thenozzle 1 are made to communicate with each other through a cooling waterpassage 5.

In the essential portion of the aforementioned transferred plasma arctorch, as shown in FIGS. 3 and 4, there is disposed in the working gaspassage 4 outside of the electrode holder 2 a swirler 6 for swirling theworking gases generally tangentially up to the tip of the electrodeholder 2.

The nozzle 1 has its tip formed with a throat 1a having a diameter d anda length 1. Inside of this throat 1a, there is formed a generallycylindrical chamber 7 which has an internal diameter D and an axiallength H and which is positioned to face the electrode 3. The internaldiameter D of the space 7 is several times as large as the diameter d ofthe throat 1a.

With the structure thus far described, the working gases flowing intothe nozzle 1 are swirled by the swirler 6 so that the swirling flowmoves downstream of the nozzle 1. In this case, the working gases haveonly the swirling velocity component at first, but their velocity isaccelerated the more as the effective area of the passage becomes thesmaller, until it takes the maximum at the position facing the electrode3. After this, the working gases pass through the chamber 7 of thenozzle 1 until they are discharged from the throat 1a.

In this instance, the axial velocity component of the working gasespassing through the chamber 7 is maintained at a small level over thedistance H of the chamber 7 and is then accelerated to the sonic levelat the throat 1a. Since, in the chamber 7, the swirling velocitycomponent is retained at the high level, there arises no problem inholding the arc stably.

In the aforementioned operations, the working gases are subjected to avelocity difference while they are passing through their passage 4 orthe gap between the nozzle 1 and the electrode holder 3, in case thecenter lines of the two members 1 and 3 are finely offset. As a result,disturbances are established in the space downstream of the lower endface of the electrode due to the asymmetrical velocity distributionsrelative to the center line of the nozzle.

However, those disturbances of the working gases are damped by theviscosity of the fluid because of the low velocity of the gases, whilethey are moving downstream at the low axial velocity component withinthe chamber 7, until the working gases restore the velocitydistribution, which is generally symmetric with respect to the nozzlecenter axis, in the vicinity of the entrance of the throat 1a.

In order to exhibit the aforementioned damping effect of thedisturbances due to the viscosity, it is necessary to reduce the axialvelocity component of the working gases to a sufficiently small leveland to retain a sufficient axial length H of the chamber 7 for dampingthe aforementioned disturbances completely.

It is, therefore, important to select the aforementioned sizes D and Hproperly.

According to our examinations, it has been found that the effects of thepresent invention can be prominently exhibited under a wide variety ofrunning conditions if the sizes D and H are determined to satisfy thefollowing relations for the diameter d of the throat 1a:

    1.5≦D/d≦10;

    and

    0.75≦H/D≦5.

FIGS. 5 to 7 show the individual essential portions of other specificembodiments of the present invention, respectively. In the embodimentshown in FIG. 5, a chamber 7a has a shape of a frustum of cone, asexpressed by D₁ >D₂. In this embodiment, too, the prominent effects canbe attained if the following relations are satisfied:

    1.5≦D.sub.2 /d≦10;

    and

    0.75≦H/D.sub.2 ≦5.

In the embodiment shown in FIG. 6, on the other hand, a chamber 7b has awarhead shape having an outer circumference formed of a smooth curve. Inthis embodiment, too, the prominent effects can be attained if thefollowing relations are satisfied:

    1.5≦D*/d≦10;

    and

    0.75≦H*/D*≦5.

In the embodiment shown in FIG. 7, moreover, a chamber 7c is formed tohave a multiplicity of steps of cylinders having their diameters reducedgradually to the outside. In this embodiment, too, the prominent effectscan be attained if the following relations are satisfied:

    1.5≦D.sub.2 /d≦10;

    and

    0.75≦H/D.sub.1 ≦5.

Here will be presented the results of comparing the example of the priorart and the specific embodiment of the present invention, as shown inFIG. 3, by comparing their operational effects:

Example of the Prior Art:

d=0.4 mm, and 1=1,4 mm;

Embodiment of the Invention:

d=0.4 mm, 1=1.4 mm,

D=2 mm, and H=3 mm; and

Operation Current - - - 12 A,

Plasma Gases - - - Oxygen Gases,

Working Gas Pressure - - - 5.5 Kg/cm², Plate to Be Cut - - - SPC, andPlate Thickness - - - 1.6 mm.

The bevel angles θ (i.e., the angles of inclination) of theabove-specified two examples are plotted in FIGS. 9A and 9B. It could befound that the cutting quality of the embodiment of the presentinvention, as shown in FIG. 9B, is better to have a smaller inclinationof cut face than that of the example of the prior art, as shown in FIG.9A, even after a long use. Moreover, another accompanying phenomenon isthat the quality of the dross to be attached to the lower end of the cutface is drastically reduced.

Incidentally, the measurements of the aforementioned angles θ ofinclination of the cut face were periodically repeated after apredetermined number of piercing.

I claim:
 1. A transferred plasma arc torch of the type, in which a tipof an electrode holder holding an electrode is enclosed by a nozzelforming a tapered working gas passage together with said electrodeholder, characterized: in that said nozzel defines a chamber of circularcross section following said tapered working gas package, said chamberhaving one end facing said electrode and another end terminating in acylindrical throat; and in that the following relations are satisfied ifthe throat diameter of said nozzel is designated at d and if thediameter and axial length of said chamber are designated at D and H,respectively:

    1.5≦D/d≦10;

    and

    0.75≦H/D≦5.


2. A transferred plasma arc torch as set forth in claim 1, comprising aswirler disposed upstream of the working gas passage inside of saidnozzel for generating a swirling flow.
 3. A transferred plasma arc torchas set forth in claim 1, characterized in that said chamber has agenerally cylindrical shape.
 4. A transferred plasma arc torch as setforth in claim 1, characterized in that said chamber generally has ashape of fustum of cone.
 5. A transferred plasma arc torch as set forthin claim 1, characterized in that said chamber has a shape having anouter circumference formed of a smooth curve, said chamber having adiameter D at said end facing said electrode and a diameter d at saidend terminating in said throat.
 6. A transferred plasma arc torch as setforth in claim 1, characterized in that said chamber has a shape of amultiplicity of cylindrical stages having their diameters reducedgradually toward said throat